Power converters such as switch-mode voltage regulators are widely used in various electronic devices for sourcing power to the electronic devices from a power source. Taking a buck type switching regulator for example, the buck switching regulator generally has relatively high conversion efficiency, wide bandwidth and good loop stability with sample loop compensation, and is thus popular in converting high input voltage to relatively low output voltage applications. FIG. 1 illustrates schematically a typical buck type DC-DC voltage converter 50. In brief, the voltage converter 50 is configured to receive an input voltage Vin at its input terminal IN, and convert the input voltage Vin into an appropriate output voltage Vo through controlling a high side switch MHS and a low side switch MLS to switch on and off in a complementary manner. The voltage converter 50 comprises a control circuit 51 for providing control signals to the high side switch MHS and the low side switch MLS. The high side switch MHS and the low side switch MLS are connected in series between the input terminal IN and reference ground GND, the common connection SW (also referred to as switching voltage output node SW) is coupled to an output terminal OUT of the voltage converter 50 via an inductive energy storage component Lo. A capacitive energy storage component Co is coupled between the output terminal OUT and the reference ground GND to smooth the output voltage Vo.
The high side switch MHS may comprise an N-channel power switching device, such as an N-channel FET or an N-channel DMOS etc. to save chip area, reduce the size and improve the performance of the voltage converter 50. In this situation, in order to make the high side switch MHS to be fully turned on (i.e., to make the high side switch MHS to operate in saturation region in which the switch MHS has a quite small on resistance), a voltage applied between a control terminal and a terminal connected to the node SW of the high side switch MHS must be large enough, at least larger than a turn on threshold voltage of the switch MHS. For instance, in the example where the high side switch is a FET/DMOS, the voltage between a gate terminal and a source terminal (connected to the node SW) of the FET/DMOS must be larger than a turn on threshold of the FET/DMOS. However, when the high side switch MHS is on, the voltage at the node SW can reach the input voltage Vin, and thus a voltage higher than the input voltage Vin must be provided to the control terminal of the high side switch MHS so as to turn it on completely.
Therefore, in order to generate a voltage higher than the input voltage Vin, the voltage converter 50 generally further comprises a bootstrap circuit 52. The bootstrap circuit 52 is configured to provide a bootstrap voltage VBST referenced to the voltage at the node SW. The bootstrap voltage VBST can be used to enhance the driving capability of the control signal DRH provided to the control terminal of the high side switch MHS, so that the control signal DRH can drive the high side switch MHS to turn on and off in good condition. In the example of FIG. 1, the bootstrap circuit 52 is illustrated to comprise a diode DB and a bootstrap capacitor CB connected in series between a bootstrap supply terminal VB and the switching voltage output node SW, wherein a cathode of the diode DB is connected to the bootstrap supply terminal VB, an anode of the diode DB is connected to a first terminal of the bootstrap capacitor CB, and a second terminal of the bootstrap capacitor CB is connected to the node SW. The bootstrap supply terminal VB is configured to receive a bootstrap supply voltage, and a voltage across the capacitor CB is provided as the bootstrap voltage VBST. The operating principles of the bootstrap circuit 52 can be easily understood by the ordinary artisan. When the high side switch MHS is turned off and the low side switch MLS is turned on, the bootstrap capacitor CB is charged by the bootstrap supply voltage till the voltage across the bootstrap capacitor CB reaches the bootstrap voltage VBST. When the high side switch MHS is turned on and the low side switch MLS is turned off, the input voltage Vin of the voltage converter 50 is transmitted to the switching voltage output node SW, i.e., the voltage at the second terminal of the bootstrap capacitor CB is pulled up to the input voltage Vin. Thus, the voltage at the first terminal of the bootstrap capacitor CB is raised to a voltage higher than the input voltage Vin, substantially equals to the input voltage Vin superposing the bootstrap voltage VBST. As the voltage at the first terminal of the bootstrap capacitor CB reaches to the input voltage Vin plus the bootstrap voltage VBST, the diode DB is reversely biased and is thus turned off so as to protect the bootstrap supply voltage source from being damaged by the relatively higher input voltage Vin.
In view of the above, it can be understood that the bootstrap capacitor CB can not be charged/recharged to refresh the bootstrap voltage VBST unless the low side switch MLS is turned on. However, in certain circumstances, the bootstrap capacitor CB may not have enough charge stored and may not be charged/recharged in time, resulting in the bootstrap voltage VBST to be decreased, which may cause the control signal DRH to not be able to drive the high side switch MHS to turn on and off properly. In such a situation, the voltage converter 50 will no longer be able to operate normally, which is not desired. For example, when the voltage converter 50 operates in light load or no load condition, the control circuit 51 is configured to reduce the on time and/or the switching frequency of the high side switch MHS and the low side switch MLS to improve the conversion efficiency of the converter 50. However, this may lead to the capacitor CB not being able to be charged/recharged in time because the on time of the low side switch MLS is too short or the high side switch MHS and the low side switch MLS do not switch in a relatively long time. In other circumstance, for example, if the desired value of the output voltage Vo is close to the input voltage Vin, the high side switch MHS has to operate in quite high duty cycle or 100% duty cycle, wherein the duty cycle refers to a percentage of the on time of the high side switch MHS in the switching cycle of the switches MHS and MLS. In this case, the on time of the low side switch MLS in one switching cycle may be quite short or the low side switch MLS may even have no chance to turn on, resulting that the bootstrap capacitor CB can not be charged/recharged in time to store enough electrical charges to provide a high enough bootstrap voltage VBST. The bootstrap capacitor CB should wait till the output voltage Vo drops, which implies that the duty cycle decreases and the on time of the low side switch MLS increases, in order to be charged/recharged so as to refresh the bootstrap voltage VBST (i.e., to make the bootstrap voltage VBST restore to a high enough value). However, this event can result in large spikes in the output voltage Vo. For example, supposing the converter 50 has a 6V input voltage Vin and a 3.3V desired output voltage Vo, a 3V bootstrap voltage VBST is required to ensure the high side switch MHS to be turned on and off normally. In this example, if an output current drawn from the converter 50 by a load is relatively small or no output current is drawn (i.e., the converter 50 operates in light load or no load condition), the bootstrap voltage VBST will decrease to lower than 2.7V, inducing the high side switch MHS not being able to turn on normally. The bootstrap capacitor CB should wait until the output voltage Vo decreases to lower than 3V in order to be recharged so that the bootstrap voltage VBST can refresh/restore to 3V. Then, the high side switch MHS and the low side switch MLS can switch normally to regulate the output voltage Vo to resume to its desired value 3.3V. However, each time the output voltage Vo resumes from lower than 3V to 3.3V, a large spike occurs, which is harmful to the converter 50 and the load, and thus is undesirable.
A need therefore exists for solving the problem of refreshing the bootstrap voltage VBST timely in power converters.